The invention relates generally to steam turbines and more specifically to maintenance operations requiring access to components within an exhaust hood of the steam turbine.
The outer shell of a steam turbine low-pressure section is generally called the exhaust hood. The primary function of an exhaust hood is to divert the steam from the last stage bucket of an inner shell to the condenser with minimal pressure loss. Usually the lower half of the exhaust hood supports an inner casing of the steam turbine and also acts as a supporting structure for the rotor. The upper exhaust hood is usually a cover to guide the steam to the lower half of the hood. The hood for large double-flow low-pressure steam turbines is of substantial dimensions and weight and usually is assembled only in the field. In many steam turbines, the inner case of the steam turbine, for example a double flow/down exhaust unit has an encompassing exhaust hood split vertically and extending along opposite sides and ends of the turbine. This large, box-like structure houses the entire low-pressure section of the turbine. The exhaust steam outlet from the turbine is generally conically-shaped and the steam exhaust is redirected from a generally axial extending flow direction to a flow direction 90 degrees relative to the axial flow direction. This 90-degree flow direction may be in any plane, downwardly, upwardly or transversely. Thus the exhaust hoods for steam turbines constitute a large rectilinear structure at the exit end of the conical section for turning and diffusing the steam flow at right angles.
The lower half of the exhaust hood, split horizontally from the upper half, directs the exhaust flow of steam to a condenser usually located generally beneath the exhaust hood. The lower exhaust hood typically supports the inner casing of the turbine and the associated steam path parts such as diaphragms and the like. The lower exhaust hood is further loaded by an external pressure gradient between atmospheric pressure on the outside and near-vacuum conditions internally. The lower exhaust hood shell is generally of fabricated construction with carbon-steel plates. Typical sidewalls for the lower exhaust hood are flat and vertically oriented. To provide resistance to the inward deflection of the sidewalls under vacuum loading, the lower exhaust hood traditionally has included internal transverse and longitudinal plates and struts. These internal transverse and longitudinal plates and struts form a web, generally underneath the turbine casing and extending to the sidewalls.
FIG. 1 illustrates typical arrangements of a prior art low-pressure double-flow steam turbine 5 with an exhaust hood 10. The exhaust hood 10 includes an upper exhaust hood 15 and a lower exhaust hood 20, mating at a horizontal joint 22. A turbine inner casing 25 is supported at multiple supporting pads (not shown) on the lower exhaust hood 20. To distribute the load from these pads to an external foundation (not shown) for the low-pressure turbine, various supporting structures are present in the form of transverse plates 40. These transverse plates 40 avoid the suction effect of the sidewalls 45 and end walls 50 and they distribute the load applied on the hood due to loads on inner casing 25. The lower exhaust hood 20 further provides a support location for shaft seals (not shown) and end bearings 75 for the turbine rotor 70. The lower exhaust hood 20 may include a framework that rests on the external foundation (not shown). The sidewalls 45 and end walls 50 may be constructed of flat metal plates, joined at seams by welding or other known joining methods. A steam inlet 30 may penetrate a top of the upper exhaust hood 15 and include a seal 55 with the upper exhaust hood. The steam inlet 30 admits steam into steam chest 35 of the turbine inner casing 25. The steam inlet 30 may be fabricated as an integral part of the inner turbine casing 25 or may be fabricated separately and welded to the inner turbine casing. Steam from steam inlet is directed by series of fixed stator vanes 60 to rotating blades 65 for driving a turbine rotor 70. Bearing housings 75 for the turbine rotor 70 are provided at axial ends of the exhaust hood 10.
When access is required to the inside of the exhaust hood 10 or inside the turbine inner casing 25, the upper exhaust hood 15 may be removed. Such access may be required for preventive maintenance, repair maintenance or modification. Due to the significant size and weight of the upper exhaust hood, a heavy-duty overhead crane is required to perform the lifting. The upper exhaust hood must be lifted high enough 88 to clear the top 31 of the highest fix object remaining attached to the inner casing 25, which is the steam inlet 30. Studies, performed to analyze construction cost of a gas turbine power plant, suggest that about $300,000 to $350,000 per meter of facility height, or up to about $10,000 per inch of facility height, is required to provide concrete block walls for such a facility in order to clear the top height.
Accordingly, it would be desirable to provide turbine equipment and methods for limiting power plant height and hence facility costs.